An electronic device provided with an electronic substrate is required to prevent defectives caused by heat from electronic components. On this account, a thermal cycle test is to be conducted before shipping. Since the temperature inside a vehicle body is greatly varied because an engine is turned on and off and ambient temperature varies, an electronic control unit for automobile use such as an ECU (Engine Control Unit) is particularly required to withstand thermal cycles (generally temperatures of −40 to 120° C. according to general rules of environmental test for automobile electronic devices JASOD001) in a wide temperature range. Recently, there is a tendency to arrange the ECU closer to the engine, and the upper limit of the temperature range tends to be more increased. When the device undergoes such thermal cycles, it is likely that the performance of electronic components in the ECU is unstable, or connection defectives are generated between a mounting board of an electronic component and the electronic component. More specifically, the electronic device is required for high heat dissipation properties, and the electronic device for automobile use such as the ECU is particularly required for more excellent heat dissipation properties than those of general electronic components.
On the other hand, an MCM (Multi Chip Module) structure is a structure that multiple LSIs are mounted on an interposer substrate (intermediate substrate) and the interposer substrate is mounted on an electronic substrate by solder bumps and the like. Since such the MCM structure (here, it is simply called the MCM structure including the structure having an interposer with no multiple LSIs) is the structure that it is difficult to release the heat generated from the LSI to the electronic substrate, it is important how efficiently heat is dissipated when the heat of the LSI is great. When the problem of this heat dissipation is solved, an electronic control unit for automobile use of the MCM structure can be implemented. For the heat dissipation structure, there is the structure that a heat sink is disposed on the top surface of the LSI, for example, but it is sometimes difficult to implement it when the thickness direction is limited because of the product structure. Furthermore, a problem sometimes arises because of reliability.
A traditional structure is described in JP-A-5-175407 and JP-A-2000-228452, which enhances the heat dissipation property of a semiconductor module using a metal core base material as an interposer substrate.
JP-A-5-175407 describes a metal core base material used for an interposer substrate. A resin layer on one side (the front surface) of the metal core base material is partially removed, and a semiconductor chip is die-bonded to an area where a core metal is exposed thus formed. Moreover, a resin layer on the other side (the back side) of the metal core base material is partially removed, and an area where a core metal is exposed thus formed is cooled by air.
On the other hand, JP-A-2000-228452 describes a metal core base material in which a semiconductor chip is mounted on one side (the front surface) in the same mounting structure as that of JP-A-5-175407. A thermal via is formed between a conductor circuit to which a solder ball for connecting another substrate on the other side (the back side) of the metal core base material and a metal core. The via hole is applied with metal plating, for example, or filled with resin and metal such as solder. It is described that such a thermal via is provided to allow efficiently dissipating the heat of electronic components. In the meantime, the structure described in JP-A-5-175407 is expected to effect heat dissipation from the part of the exposed metal core on the back side of the metal core base material to ambient air, but the product structure having an underfill and gel, for example, where an air flow hardly occurs is rarely expected to effect this advantage. Furthermore, since the difference in thermal expansion coefficients between the interposer substrate and the motherboard is not considered, it is likely to generate connection defectives caused by thermal cycles.
Moreover, in the structure described in JP-A-2000-228452, the thermal via is formed from the core metal to the conductor circuit, but it is costly to form the thermal via. Besides, the thermal via that can be formed in a printed wiring board generally has poor heat dissipation properties because of a hollow, thin plate by copper plating.